Lightguide preforms are substantially cylindrical boules of glass having an inner glass core surrounded by a glass cladding having a lower index of refraction. Such a preform is heated in a furnace and a lightguide fiber drawn from the reflowed portions thereof. There are several well known methods for fabricating such preforms. One technique referred to as the outside vapor deposition (OVD) process is disclosed in U.S. Pat. No. 3,737,292. In this process, glass particles, referred to herein as soot, are generated by a flame hydrolysis burner. The soot is directed onto a cylindrical mandrel and a plurality of layers of the soot deposited thereon by providing a relative lateral motion between the burner and the mandrel. The composition of the layers may be changed to alter the refractive indices thereof. After a sufficient number of layers have been built up, the mandrel is removed and the soot is consolidated in a dehydrating atmosphere to form a dense, low OH, solid cylindrical article free from particle boundaries. The central opening remaining from mandrel removal may be eliminated in a number of ways as disclosed in U.S. Pat. Nos. 4,157,906; 4,251,251; 4,344,670 and 4,358,181. The resultant composite preform can be heated and drawn, with or without the central opening, directly into an optical fiber.
A further technique for manufacturing a lightguide preform is referred to as the integral mandrel process. In this process a glass mandrel, which becomes an integral part of the preform, is used. The glass mandrel is placed in a lathe and rotated as layers of the glassy soot are deposited thereon. The glass mandrel, with the soot cladding thereon, is removed from the lathe and placed in a furnace to consolidate the soot on the mandrel to fabricate the preform. One example of this technique referred to as the hybrid process is disclosed in U.S. Pat. No. 4,257,797 and is hereby incorporated by reference herein.
Although this process has been found to be effective, bubbles often occur at the soot-mandrel interface particularly near the ends of the resulting preform. These bubbles are believed to be the result of density variations in the soot caused by overheating near the ends of the preform due to the relative reciprocal motion of the boule past the torch. Such defects will deleteriously effect the transmission characteristics and strength of lightguide fiber drawn from such a preform.
Accordingly, there is a need for an integral mandrel process wherein bubbles formed at the soot-mandrel interface are substantially eliminated.